Time travel is impossible. Therefore, Omega's "going back in time" to kill your grandfather was actually just it predicting you taking both boxes and killing him based on that prediction. Or, if Omega comes from outside of spacetime and just interacts with our universe by manipulating the universal wavefunction, it could at most kill your grandfather from another Everett branch, since the one from yours definitely survived.
In the former case, yes, your existence is evidence that Omega predicted you would just take box B. In the latter case, it's best to act as if that's what Omega predicted anyway.
Nobody has any idea how accurate Omega's predictions are.
If you interpret that as a uniform prior between 0% accurate and 100% accurate, then Omega's "prediction" is a coin toss. If Omega's predictive accuracy is something you have no other reason to care about, take both boxes.
However, being the sort of person who takes the box makes you less likely to exist in worlds where this prediction is accurate, and more likely to exist in worlds where the prediction is inaccurate. So if you think that Omega's predictive accuracy on this question is correlated with his predictive accuracy on other questions, and you strongly want future predictions to be accurate, don't take box A.
It seems odd to describe something as a prediction when you believe it uncorrelated with the outcome. Suppose you think Omega's predictions are somewhere between 50% and 90% accurate.
Being the sort of person that takes both boxes makes you less likely to exist. So don't take box A unless you think that the universe you are in is exceptionally bad and want to destroy it.
Omega is almost certainly lying about the going back in time to kill your grandfather, and given that nobody has any evidence of their predictive ability, probably terrible at that too. They're also the sort of person to openly threaten murder.
On the whole, Omega is probably someone that you want nothing to do with, so just smile without showing your teeth while slowly disengaging until it's safe to get out of there more rapidly.
In front of you are two boxes.
Box A contains $1000.
If Omega predicted you would just take Box B, Box B contains $1,000,000.
If Omega predicted you would take both boxes, Box B contains nothing. Omega will also go back in time and kill your grandfather.
Nobody has any idea how accurate Omega's predictions are.
Does the fact that you exist prove that Omega thought you would just take one box, so Box B definitely contains the million dollars, and if you took both boxes it would imply that Omega was a terrible predictor so you might as well just take both boxes and run?